Friday, 20 July 2018

Electrical Safety and OISD 234

Electrical Safety and OISD -234
Safety in Electrical System deserves maximum attention especially in the hydrocarbon industry, where electricity constitutes one of the major sources of ignition that could cause a fire or an explosion.

Hazardous Area classification shall be followed and should be carried out in line with OISD-STD-113 and IS: 5572.  Accordingly, the electrical equipment / fittings should be selected in line with IS: 5571.
Area classification:
ZONE - 0: An area in which a flammable atmosphere is continuously present.
ZONE-1: An area in which a hazardous atmosphere is likely to occur under normal operating condition.
ZONE-2: An area in which a hazardous atmosphere is likely to occur only under abnormal operating conditions likely to occur under normal operating condition.

Note: Approval from DGMS(Directorate General of Mines Safety  ) shall be available for equipment used in hazardous area


Earthing system design shall be carried out in accordance with the requirements of CEA regulations 2010 and IS:3043.
i) Earthing system in general, shall cover the following:
- Equipment earthing for personnel safety,
- System neutral earthing, and
- Static and lightning protection.
ii) The earthing system envisages an earthing network with designed number of earth electrodes attached to it. The following shall be earthed:

Earthing coverage
a)      Metallic non-current carrying parts of all electrical apparatus such as   transformers, switchgears, motors, lighting/power panels, terminal boxes, control stations, lighting fixtures, receptacles etc.
b)      Current and potential transformer secondary neutral    System Neutral
c)      Steel structures, Loading platform etc.
d)     Metallic Cable trays and racks, lighting mast and poles.
e)      Storage tanks, spheres, vessels, columns and all other process equipment.
f)       Electrical equipment fencing (e.g. transformer, yard etc.)
g)      Cable shields and armour
h)      Flexible earth provision for Tank Wagon, Tank Truck

Earth resistance value

The maximum resistance value of an earthing system to the general mass of earth should not exceed:
a)      4 ohms for electrical systems and metallic structures.
b)      7 ohms for storage tanks.
c)      1 Ohm for main earth grid

Minimum Size of earthing conductors for electrical equipment
Sr. No.
Type of equipment
Earth Conductor Size
Building Columns
40mmx5mm GI strip
Storage tanks (vertical &horizontal)
40mmx5mm GI strip
Loading racks
40mmx5mm GI strip
Pipe racks , vessels, & heat exchanger
40mmx5mm GI strip
Motor upto 3.7KW
8 SWG solid GI wire
Motor upto 5.5KW30KW&welding receptacle
10 mm dia GI wire rope
Motor upto 37KW and above including HV motors
10 mm dia GI wire rope or 40mmx5mm GI strip
Small equipment of instrument
8 SWG solid GI wire /rope
Push button station
8 SWG solid GI wire
Lighting, power instruments, panels
10 mm dia GI wire rope
Street light poles
10 mm dia GI wire rope
Lighting transformer
16 mm dia GI wire rope
Bonding of pipe
25 sqmm insulated flexible copper cable
MV/HV bus duct, power transformer
As per calculation
HV&EHV substation
As per calculation
LA (5KA,9KA)         
25×3 mm Cu Strip
HT Switchgear
50×6 mm GI Strip
50×6 mm GI Strip
Fence / Rail Gate
50×6 mm GI Strip
Cable tray
50×6 mm GI Strip


The protective system shall be selected and coordinated to ensure the following:
a. Protection of equipment against damage which can occur due to internal or external short circuits, overloading, abnormal operating conditions, switching, lightning surges, etc.
b. The continuity of operation is maintained to those parts of the system not affected by the fault.
c. Personnel and plant safety-
Relays  and protective devices shall be suitably selected and coordinated.
ii) Recommended relay protections for Transformers, motors and feeders which are generally encountered in distributing network.
-Electrical loads essential for the safe shutdown of the plant.
- Emergency lighting, security lighting, obstruction lights.
- Process plant instruments as required.
- Communication equipment, Fire Alarm control panels.
- Firefighting equipment excluding main fire water pumps.
- Loads critical for process, plant and personnel safety.
Note: within a period not exceeding 30 seconds from the instant of failure of normal supply
Critical Power Supply Systems- UPS or DC Power Supply
in the event of Normal/ Emergency supply failure –
- Critical Lighting system
- Critical instrumentation and process control,
- Critical communication  like telephone, walky-talky (VHF) equipment,
- Fire and Gas detection system
·         Sufficient space in  front  and back to allow access for cable terminations , maintenance and inspection of equipment.
·         Cable trenches, wherever provided, inside sub-stations shall be filled with sand and to be shield.
·         Fire extinguishers, suitable for electrical fires (CO2 type) and round bottom fire buckets with clean dry sand.
·         Each substation building shall have at least two doors.
·         to locate DG sets in a separate house/shed away from Substation in a safe area.
·         adequate clearances between equipment, and equipment.
·         The battery room shall be a separate and freely ventilated room.
Note: the battery room and walls up to height of one meter shall have acid resisting material/coating.

¨  should preferably have winding temperature indicators with alarm / trip provisions.
-To ensure maximum safety during operation, inspection.
-Switchboard shall be totally enclosed, dust and vermin proof.
-Barriers shall be provided to permit personnel to work safely.
-Suitable interlocks .
-Insulating mats of appropriate voltage level (conforming to IS-15652.
-Panels shall be painted with the description of its identification at front and at the rear.
-Starting of motors from substation shall not be allowed.
-All H.V. switchgears should have provision of view glasses/ IR windows.


i) Ingress protection for motor should preferably be IP 44 for indoor use and IP 55 for outdoor duty motors.
ii) The permissible noise as per Pollution Control Board (CPCB) guidelines.
iii) winding insulation shall be class F and temperature rise limited to that specified in the applicable IS for class B insulation .


i) shall be designed to start automatically on power failure and feed the selected loads.
ii) Diesel Engine installation, do not call for Area Classification, provided the D.G. room is properly ventilated.
ii) DG sets shall comply with the latest guidelines of environment ministry with regard to noise levels.
iii) OISD STD 127 to be referred for details on operation, inspection & maintenance of Diesel  Engines.


i) Each Motor shall be provided with a control station in the field, a STOP push button as a minimum.
ii) Enclosure shall have suitable protection as per hazardous area classification.
iii) It includes individual process requirements:  - Start/stop push button, close/ neutral/, Trip Switch, Ammeter.
Iv) Local/Remote Selector Switch.
V) Auto/Manual selector switch and cable glands.

Plant lighting
i) In addition to the normal lighting, each installation shall be equipped with emergency and critical lighting.
ii) Critical lighting shall enable safe evacuation of operating personnel and shall be employed along escape route, assembly point and critical installations such as first aid centre, control rooms, manned sub-stations, fire water pump house etc.
iv) For hazardous areas, emergency lighting fixtures shall be explosion proof Ex(d) type, irrespective of the area being classified as zone-1 or zone-2.
v) Voltage 250V, if the neutral or the midpoint of the system is connected with earth.


i) The statutory clearances from the State Electrical Inspectorate/Director General, Mines Safety.
ii) Manufacturer’s test certificates shall be obtained (for the successful passing of shop tests.
iii) Installations conform to acceptable engineering standards and are appropriate to the respective hazardous area classifications.

- Electrical checks/ testing as recommended by OEM
- Condition of terminal box: tightness of nuts & bolts, missing bolts, condition of bushings.
- Foundation bolts –Tightness
- Check terminal bushings for any damage or looseness 
- Checking of exciter/field/stator coils, AVR, Rectifier Bridge, brushes, surge suppressor or variable resistor (varistor) etc.
- Healthiness of protection circuits.
- Proper functioning of Space Heaters.
- Protection circuits (testing and simulation tests)
- Auto/manual starting and changeover system
- Earthing connection and record Insulation Resistance
- Measure & record PI (polarization Index) value, greater than 2.
- Earth resistance value & test date on the earth pit cover.

Electric Motor
-Electrical checks/ testing as recommended by OEM.
- Condition of terminal box: nuts & bolts tightened, missing bolts.
- Cable termination (size and type of cable).
- Driving end condition:
- Non driving end condition:
- Foundation bolts -Tightness
- Insulation Resistance value
- Space heater where provided.
- Motor on no-load test 

-Electrical checks/ testing as recommended by OEM
- All incomer and outgoing, bus bar, support insulators, phase barriers
- Functionality check/ testing of CBs, MCCBs, CT, PT, Bus Bar, protection relays, Energy meters, Power Packs etc.
- Operation of interlocks and safety devices.
- Insulation resistance measurement for Bus Bar, power cables, Breakers.
- Switchgear panel earthing by two separate and distinct connections to earth.
- Primary injection testing of switchgear Incomer and Outgoing panels.
- Secondary injection testing of protection relays
- Electrical & manual closing and tripping of all circuit breakers (remote and local)
- Check Circuit Breakers for proper racking-in and rack-out operation.

-Electrical checks/ testing as recommended by OEM.
- Body and neutral earthing/ earth resistance
- Check insulation resistance and PI of HV and LV windings.
- Dielectric strength of Transformer oil (as per IS : 335)
- Oil level in the conservator, bushings and thermometer pockets
- Operation of the tap changer circuits on all the tap positions (simulation)
- Functioning of Buchholz relay
-Checking/ testing of marshalling panel
-Oil/ winding alarm/ trip ckt. simulation.
-Protection system checking.
- Cooling system (automatic starting and stopping of fans)
- Sealing of bus duct entries through walls (for vapour, dust, water tightness)

1. In the loading and unloading area all pipe-lines, fittings and delivery hoses or metal pipes, metallic loading arms, swivel joints, tanks, chassis of tankers shall be electrically continuous and be efficiently earthed.
2. No mechanically propelled tankers shall be loaded or unloaded until its engine has been stopped and battery isolated from electrical circuit.
3. The engine shall not be restarted and the battery shall not be connected to the electrical circuit until all tanks, and valves have been securely closed:
4. Adequate Fire-Fighting equipment shall be kept readily available during loading of tankers for immediate use


i. Electrical Caution Board on electrical equipment when under maintenance or repair
ii. Notice for restricted entry in Electrical control room / Substation

-Insulating mats conforming to IS:15652
- Safety Shoes
- Fuse handling equipment
- Goggle/ Face Shield
- Electrical discharging/ Earthing Rod
- Gas masks (for generating station with capacity 5MW or above)
- Safety helmet
- Safety Belt
- Safety Torch

i. Single line diagram
ii. Hazardous area classification of the location
iii. Electrical Shock Treatment Chart
iv. Important / emergency telephone numbers
v. Isolation / lock –out tags
vi. Escape route


i. Attendance Register
ii. Shift Roster
iii. List of trained First Aid & Firefighting personnel
iv. List of persons with Electrical License (wiremen & supervisory)
v. Accident Records
vi. Log Book & equipment maintenance record
vii. Daily check list of electrical equipment
viii. Earth Resistance & Insulation Resistance Record
ix. Work Permits
x. Lay out Drawings
xi. Equipment/Earthing/Cable

BIS (Bureau of Indian Standards) Publications
* IS 5571 Guide for selection of electrical equipment for hazardous areas.
* IS 5572 Classification of Hazardous area having flammable gases and vapour for electrical installation.
* IS 7820 Electrical Apparatus for Explosive Gas Atmospheres - Method of Test for Ignition Temperature
* IS 3043 Code of Practice for Earthing.
* IS 2309 Code of practice for the protection of buildings and allied structures against lightning
* IS 15652 Insulating mats for electrical purposes
* DGMS letter No. 1 (6) 2001- Genl / 3604 -3753
*CA regulation 2010

Work Permit System shall be followed as per OISD STD 105 .
OISD 234

1. Proper functioning of electrical equipment can only be ensured by means of periodic preventive and predictive maintenance of the equipment. This enhances equipment life and also ensures safety of the equipment, installation and operating personnel.
2. Maintenance may be daily, weekly, quarterly or annual depending upon the type of equipment. Adequate logs shall be maintained to ensure that maintenance is carried out as per approved checklists. Preventive maintenance should be carried out as per schedules laid down in OISD STD- 137.
3. Live parts of switch gear shall be made inaccessible from any inadvertent physical contact, lizard entry etc. These shall be suitably covered by barriers and shrouds.
4. It shall be ensured that the electrical installation shall be rust / corrosion protected. This should be achieved by painting all equipment at regular intervals by use of zinc passivated, nickel plated hardware, stainless steel hardware, through provision of canopies for outdoor equipment and covering of terminal boxes to prevent ingress of water etc. Battery rooms
shall be kept dry and well ventilated and all terminals shall be kept covered with petroleum jelly to avoid corrosion.
5. During maintenance if any equipment is removed from panel doors, etc. which leaves an open cutout on the equipment, the same shall be promptly sealed using blanking plates or
other means, to ensure that there is no loss of the degree of ingress protection and also that this does not provide a means of access for entry of dust / vermin etc. It shall be ensured that all unused cable entries are blanked off.
6. All battery banks shall be routinely checked for healthy cell voltage, specific gravity of cells, electrolyte level etc.
7. Insulation resistance of all electrical equipment, such as, all switchboards, motors, transformers, cables etc. shall be routinely measured and logged to ensure healthiness of equipment.
8. Dielectric strength of transformer insulating oil, Oil Circuit Breakers etc. shall be measured at regular intervals and recorded and oil filtration shall be carried out wherever necessary. Dissolved Gas analysis for transformers rated 5MVA and above may be carried out as a predictive method to assess health of transformer.
9. All cable terminations, bus-bar joints, etc. shall be tightened, more so for equipment subjected to vibrations, to ensure that there are no hot spots which could lead to fire / equipment failure.
10. Settings of protective relays shall be checked to ensure that they are set as per the recommended settings. Protective Relays shall be tested / calibrated once a year to ensure proper operation as per manufacturer’s recommendations.
11. Unused cable entries in any switchboard / Electrical installation shall not be kept open.
12. Conduits used for cable entry shall be sealed and earthed.
13. All wall openings of an electrical substation including cable entries shall be properly sealed
to arrest water entry inside substation.
14. Transformer and switchyards shall be maintained free of vegetation / dry grass.
15. Earth grid resistance shall be measured and recorded regularly to ensure operation of related protective devices in case of fault.
16. All electric apparatus and wiring in a hazardous area shall at all times be so maintained as to retain the characteristic on which their approval has been granted
17. Equipment enclosures and fittings shall be examined to see that all stopping plugs and bolts are in position and properly tightened. Locking and sealing devices shall be checked to ensure that they are secured in prescribed manner.
18. Replacement fasteners, nuts, studs and bolts shall be of the type specified by the manufacturer for the particular apparatus. No attempt shall be made to replace or repair a glass in a flameproof enclosure e.g. in a luminaire or other enclosures, except by replacement with the complete assembly or part obtainable from the manufacturer, complying with the approval certificate.
19. If at any time, there is a change in the area classification or in the characteristics of the flammable material handled in the area or if the equipment is relocated in the area, the area classification drawing should be correspondingly revised and a check shall be made to ensure that the equipment selection corresponds to the revised area classification.
20. A system shall be established to record the results of all inspections and the action taken to correct defects.

No comments:

Post a Comment